1. Field of Endeavor
The present invention relates to dielectrophoresis and more particularly to a three dimensional microelectrode system for dielectrophoresis.
2. State of Technology
Dielectrophoresis (DEP) is a force that is proportional both to the volume of objects/particles and to the gradients of electric potential energy on those objects/particles. For a spherical particle of radius r, the force on the particle in a fluid medium depends on the Clausius-Mossotti factor (CM); a ratio of the conductivities and permittivities of the surrounding medium and the particle itself. The direction of the force depends on the sign of the CM and can change sign as a function of the electric field frequency. When the CM is positive, particles will be attracted to regions of relatively high electric field (known as positive DEP), and when the CM is negative, particles will be repelled from regions of high electric field (negative DEP), with the force being proportional to the local gradient of the electric potential energy.
DEP has been used to extensively study the material properties of biological samples, the force exhibited by swimming bacteria, and as a method for particle separation. With few exceptions, DEP has been performed on samples with low surrounding medium conductivity (10^−5-10^−1 mS/cm). There are many reasons for this, with varying degrees of importance. At higher conductivity, the CM is almost always negative, which makes separation based on the sign of the CM impossible, as all particles will travel away from regions of high electric field. Additionally, applying electric fields through highly conductive media can lead to several extremely disruptive effects, such as solution joule heating, bubbling from electrolysis, loss of voltage over the electric double layer, and the degradation of the electrodes used to drive the field gradients. Finally, the increased solution conductivity can reduce the resistance of the DEP system, which can lead to a significant loss of voltage in the region of interest as a greater portion of the voltage is dropped over the wires leading to the electrodes or power supply itself due to its internal resistance. Since the DEP force scales with the square of the electric field, reducing the effective voltage due to these system losses reduces the force on particles. However, many biological samples of interest naturally possess solution conductivities between 1 and 20 mS/cm, meaning that for DEP to be effectively applied to these samples in traditional systems there must either be a dilution or other sample processing step. This can lead to damage of the biological particles of interest, either due to mechanical reasons, or due to the loss of osmotic pressure from the lowered solution conductivity. Additionally, these processes require some degree of sample handling which increases time, cost, and required operator skill level in a laboratory or clinical environment. Ideally, Applicants would like to have a sample analysis system that is minimally disruptive to cell populations, and can be done in an automated format with little to no preparation.